In a year when the Arctic ice cap has shrunk to the lowest level ever recorded, a new analysis from Seattle scientists says global warming will accelerate future melting much more than previously expected.

About 40 percent of the floating ice that normally blankets the top of the world during the summer will be gone by 2050, says James Overland, an oceanographer at the National Oceanic and Atmospheric Administration's Pacific Marine Environmental Laboratory. Earlier studies had predicted it would be nearly a century before that much ice vanished.

"This is a major change," Overland said. "This is actually moving the threshold up."

The finding, to be published Saturday in Geophysical Research Letters, adds to a growing body of evidence that the ecosystem around the North Pole is rapidly transforming, says Mark Serreze, of the National Snow and Ice Data Center at the University of Colorado. He goes even further than Overland, predicting the Arctic Ocean will be completely ice-free in summer by 2030.

"If you had asked me a few years ago, I would have said it wouldn't happen until 2070 or 2100," said Serreze, who was not involved in Overland's project.

Even a 40 percent loss of ice would be devastating to ice-dependent animals such as walruses and ringed seals, said Overland, who shared his data with federal officials considering an endangered-species listing for polar bears.

Gray whales will suffer if the ice-loving crustaceans they feed on disappear. But some commercially important fish species, like pollock and salmon, could thrive in warmer water — a possible boon for the Seattle-based fishing fleet that plies Alaska's Bering Sea. There are also hints, though, that the disappearance of ice would favor predators that undermine fisheries, Overland said.

Shipping will benefit if the Northwest Passage across the Canadian Arctic melts out each summer — as it did for the first time this year.

An international power struggle already is under way as governments rush to stake claims on territory, oil deposits and mineral resources that will become accessible as the Arctic ice cover shrinks.

A Russian submarine planted a flag on the sea bed under the North Pole this summer. Canada announced plans for military bases along the Northwest Passage, which it claims as sovereign territory. President Bush insists the route is an international waterway.

"There will be winners and losers," Overland said.

Melting of the Arctic ice cap does not contribute to sea level rise. Floating ice, like that around the North Pole, already displaces water, like ice cubes in a glass of tea. When the ice melts, sea level remains unchanged, though salinity can be altered. The major sea level rise associated with global warming would come if the massive ice sheets covering Greenland and Antarctica melt and flow into the oceans.

Overland and Muyin Wang, a meteorologist at NOAA's Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington, analyzed 20 computer climate models used by the Intergovernmental Panel on Climate Change (IPCC). The international body issues periodic assessments of global warming and its future impacts. The IPCC's 2001 assessment predicted major Arctic ice loss wouldn't occur until 2100.

The Seattle team used the computer models to predict historic ice levels, and compared the results with actual measurements. They eliminated about half the models because they didn't do a good job of matching the data. Then, they used the best models to look ahead.

Their findings echoed those of the IPCC's most recent report, issued earlier this year. But the new results are more solid because the unreliable models were weeded out.

"We've reduced the uncertainty," Overland said. "We're absolutely going to lose major ice before 2050."

But even the best models can't account for the rapid changes seen over the past few years, Serreze pointed out.

In an average August between 1979 and 2000, the Arctic Ocean was covered with about 3 million square miles of sea ice, according to the National Snow and Ice Data Center. By Labor Day this year, the total had shrunk to a little more than half that, shattering the previous record low set in 2005.

"Usually when you break a record it's by a few percentages," said William Chapman, who monitors the Arctic for the University of Illinois, and was one of the first to point out this year's "shocking" melt. "This year we blew right through it."

Scientists blame the rapid retreat on a combination of natural weather fluctuations and global warming. Nature could conceivably cool things off again in the short term, Overland said. But over the long run, global warming will dominate.

And because greenhouse gases linger in the atmosphere for up to five decades, the melting will intensify even if emissions from cars, power plants and industries are slashed dramatically.

"I'm afraid to say a lot of the impacts we're going to see in the next 30 to 40 years are pretty much already established," Overland said.

Reductions in greenhouse gases could begin to have a moderating effect by the next century.

Sandi Doughton: 206-464-2491 or sdoughton@seattletimes.com